1. Technical Field
The present invention relates to wireless communications, and, more particularly, to conducting packet-data communications via wireless networks.
2. Description of Related Art
a. CDMA Networks Generally
Many people use mobile stations, such as cell phones and personal digital assistants (PDAs), to communicate with cellular wireless networks. These mobile stations and networks typically communicate with each other over a radio frequency (RF) air interface according to a wireless communication protocol such as Code Division Multiple Access (CDMA), perhaps in conformance with one or more industry specifications such as IS-95 and IS-2000. Wireless networks that operate according to these specifications are also referred to as “1 xRTT (1x) networks,” which stands for “Single Carrier Radio Transmission Technology”. These networks (referred to herein as “CDMA networks”) typically provide communication services such as voice, Short Message Service (SMS) messaging, and packet-data communication.
Typical CDMA networks include a plurality of base stations, each of which provide one or more wireless coverage areas, such as cells and sectors. When a mobile station is positioned in one of these coverage areas, it can communicate over the RF air interface with the base station, and in turn over one or more circuit-switched and/or packet-switched signaling and/or transport networks to which the base station provides access. The base station and the mobile station conduct these communications over a frequency known as a carrier. Note that base stations may provide service in a coverage area on one carrier, or on more than one.
Communication, including packet-data communication, between the mobile station and the base station is separated into forward-link communication (from the base station to the mobile station) and reverse-link communication (from the mobile station to the base station). And each carrier over which this communication takes place is typically actually a pair of distinct frequencies—one for the forward link and the other for the reverse link. This approach is known as frequency division duplex (FDD).
In a typical CDMA network, using a configuration known as radio configuration 3 (RC3), a base station can, on each carrier in each sector, transmit forward-link data on a maximum of 64 distinct channels at any given time. As a side note, an instance of a carrier in a sector may be referred to herein as a “sector/carrier.” Each of these 64 channels corresponds to a unique 64-bit code known as a Walsh code. Of these, typically, 61 channels are available for use as traffic channels (to carry user data), while the other 3 are reserved for administrative channels known as the pilot channel, the paging channel, and the sync channel.
When a base station instructs a mobile station to use a particular traffic channel for a particular communication session, the base station does so by instructing the mobile station to tune to a particular one of those 64-bit Walsh-coded traffic channels. It is over that assigned traffic channel that the base station will transmit forward-link data to the mobile station during the ensuing communication session. Note that, in addition to including the forward-link channel, the traffic channel also includes a corresponding Walsh-coded reverse-link channel, over which the mobile station transmits data to the base station.
These traffic channels may be used for different types of communication, among which are second-generation (2G) voice, 2G data, third-generation (3G) voice, and 3G data. 2G voice is circuit-switched, which involves using an assigned traffic channel for the duration of a call, and is conducted at a data rate of 9.6 kilobits per second (kbps). 2G data is also circuit-switched, somewhat analogous to a dial-up connection over a telephone line between a personal computer and a modem pool, and is conducted at a data rate of 14.4 kbps. Like 2G voice, 3G voice is circuit-switched and is conducted at a data rate of 9.6 kbps. Finally, 3G data is packet-switched, which involves using a traffic channel during actual data transmission and not during so-called “downtime,” and, if conducted only over a single 64-bit-Walsh-coded traffic channel (more on this below), is conducted at a data rate of 9.6 kbps.
b. 3G Data: Fundamental and Supplemental Channels
When a mobile station requests a traffic channel to engage in 3G data communication, the 64-bit-Walsh-coded forward-link channel that the base station initially assigns to the mobile station is referred to as a fundamental channel (FCH). It is often the case, however, that the mobile station requests download of more data than can be transmitted to the mobile station over the FCH in an elapsed time that will be satisfactory to the average user. In that case, assuming that all 61 of the base station's 64-bit traffic-channel Walsh codes on that sector/carrier are not occupied by other mobile stations, the base station can request that the mobile station accept data on what is known as a supplemental channel (SCH). As stated, the maximum achievable data rate using the one 64-bit Walsh code corresponding to the FCH is 9.6 kbps; however, higher data rates can be achieved on an SCH.
Like an FCH, an SCH corresponds to a single Walsh code on which the base station instructs the mobile station to receive 3G data. However, the Walsh codes used for an SCH are typically shorter in bit-length than the 64-bit Walsh codes that are used for an FCH. It is by using these shorter-bit-length Walsh codes that higher data rates are achieved on an SCH. However, the availability of these shorter-bit-length Walsh codes depends on how many of the sector's 64-bit, traffic-channel-dedicated Walsh codes are not in use at the time on the carrier in question. The reason for this is that each shorter-bit-length Walsh code corresponds directly to—and essentially occupies—a particular set of the sector/carrier's 64-bit Walsh codes.
Specifically, an SCH can achieve a data rate of 19.2 kbps by using a 32-bit Walsh code, which occupies a particular set of two of the 64-bit Walsh codes on the sector/carrier. The two 64-bit Walsh codes are occupied in the sense that, while the base station is using that 32-bit Walsh code to transmit data to a mobile station on an SCH, the base station is not able to instruct other mobile stations to use either of those two 64-bit Walsh codes.
Similarly, an SCH can achieve 38.4 kbps by using a 16-bit Walsh code, which occupies a particular set of four of the base station's 64-bit Walsh codes on that sector/carrier. As a further example, an SCH can achieve 76.8 kbps by using an 8-bit Walsh code, which occupies a particular set of eight 64-bit Walsh codes. As a final example, the SCH can achieve 153.6 kbps by using a 4-bit Walsh code, which occupies a particular set of sixteen 64-bit Walsh codes.
When a base station has enough data to warrant using an SCH for more than one mobile station on a given sector/carrier, the base station makes use of an entity known as an “RF scheduler” to place those mobile stations in a queue (the “SCH queue”). Each mobile station in the SCH queue is then sequentially given a turn to receive data over the SCH, each turn limited in duration by a base-station parameter known as the “SCH burst duration.” Thus, one by one, the mobile stations in the SCH queue receive data over the SCH in bursts that each last a period of time equal to that parameter, a typical value for which is 320 milliseconds, and which is usually set by either the base-station manufacturer or by the owner/operator.
If one burst is not enough to transfer all of the data that the base station has for a given mobile station, the base station will put the mobile station back in the SCH queue and, when its turn comes up again, send another request to the mobile station to again tune to the SCH and receive another burst. The mobile station then accepts this request and receives another burst. This process repeats until the base station has transmitted all of the data to the mobile station. Thus, any periods of time during which the mobile station is waiting in the SCH queue for the base station to request that the mobile station receive another burst on the SCH will reduce the effective data rate at which the mobile station is receiving data.
Note that, in some implementations, mobile stations that are receiving data on the SCH can also receive data on the FCH. In other implementations, mobile stations that are receiving data on the SCH can not also receive data on the FCH. In still other implementations, mobile stations that are receiving data on the SCH may receive overhead data, signaling data, control data, and/or other types of administrative data on the FCH.
c. Forward-Link Transmission-Power Management
i. Forward-Link Frame Error Rate (FFER)
In CDMA networks, the transmitting power of a base station on a given sector/carrier is divided among the mobile stations to which the base station is transmitting voice data and/or packet data on traffic channels, as well as among the pilot, paging, and sync channels mentioned above. With respect to mobile stations that engage in 3G voice and 3G data communications, the amount of power that the base station allocates to the transmission to any one mobile station is based on a number of factors, one of which is known as the forward-link frame error rate (FFER). Note that, in CDMA networks, data is transmitted from the base station to the mobile station (and vice versa) in data units that are known as frames.
Some of the frames received by mobile stations contain errors as a result of imperfect transfer from the base station, while some do not. The FFER is a ratio of the number of error-containing frames that the mobile station receives to the total number of frames that the mobile station receives, over a given time period. Note that the FFER calculations often also take into account frames that are not received at all by the mobile station. And, other things being more or less equal, the more power that the base station allocates to a given mobile station, the lower the mobile station's FFER will be. In operation, a mobile station reports its FFER to the base station, and the base station adjusts the power allocated to that mobile station accordingly. This back-and-forth calibration is conducted in an attempt to keep the mobile station's FFER at or below what is deemed to be an acceptable threshold, which typically will be around 2%.
More particularly, the mobile station periodically (e.g. once every 100 or 200 frames) computes its FFER, and reports it to the base station. The base station then adjusts its transmission power accordingly for that mobile station's assigned traffic channel. If the FFER is too high with respect to what is deemed to be an acceptable threshold, the base station increases transmission power in an effort to reduce the FFER. If the FFER is below the threshold, the base station may allocate less power to that mobile station, to have more available for other mobile stations. Again, this process is conducted in an attempt to keep the mobile station's FFER at or just below the acceptable threshold, often referred to as the “FFER target.”
Note that different situations may present themselves on a given sector/carrier at different times. For one, the number of mobile stations using FCHs can vary between just a few, such as 10, to a larger number, such as 30, and perhaps approach the upper bound of 61 (assuming RC3). And, as stated, the power that the base station allocates for transmission to these mobile stations can vary. In particular, variables such as terrain, weather, buildings, other mobile stations, other interference, and distance from the base station can affect the FFER that each mobile station reports, and thus the amount of power the base station allocates for each mobile station. Since base stations have a finite amount of power that they can allocate to the mobile stations on a sector/carrier, increasing the transmission power to some or all of those mobile stations (to keep their FFERs low) generally results in the base station being able to serve fewer mobile stations on that sector/carrier. That is, it reduces capacity on the sector/carrier.
ii. The Logarithmic Ratio Ec/Ior 
As explained, in CDMA networks, a given base station has a finite amount of power for transmitting on each sector/carrier on which it is providing service. The base station divides this power among any active traffic channels (over which it is transmitting voice and/or packet data to mobile stations), as well as among the pilot, paging, and sync channels. Periodically, for a given sector/carrier, the base station calculates a ratio of (a) the power it is allocating for transmitting the pilot channel (the “pilot-channel power level”) with (b) the power it is allocating for transmitting all (i.e. pilot, paging, sync, and traffic) channels (the “all-channel power level”).
This ratio is a base-10 logarithmic one, and is known as “Ec/Ior.” The pilot-channel power level is referred to as “Ec”—“energy per chip.” The all-channel power level is referred to as “Ior”. Ec and Ior can each be expressed in Watts (W), milliwatts (mW), or any other suitable units of measure. Note that Ec and Ior are often expressed as base-10 logarithmic ratios themselves, with respect to a reference power level of 1 mW. In that case, Ec and Ior would each typically be expressed using the unit “dBm,” where “dB” indicates decibels and “m” indicates the reference power level. So, Ec can be expressed as the base-10 logarithmic ratio of the pilot-channel power level (in mW) and 1 mW. And Ior can be expressed as the base-10 logarithmic ratio of the all-channel power level (in mW) and 1 mW.
Ec/Ior is typically expressed as the base-10 logarithmic ratio of the pilot-channel power level and the all-channel power level, each of which may be measured in Watts. As such, the typical unit of measure for Ec/Ior is the decibel (dB). As an example, if a base station were allocating about 2 W (2000 mW) for the pilot channel, Ec would be about 33 dBm, calculated as 10*log((2000 mW)/(1 mW)). And if the base station were allocating a total of about 10 W (10,000 mW) for the pilot, paging, sync, and active traffic channels, Ior would be about 40 dBm, calculated as 10*log((10000 mW)/(1 mW)). In this example, Ec/Ior would be about −7 dB, calculated as 10*log((2 W)/(10 W)). Note that Ec/Ior will always be negative, as long as at least some power is allocated for any one or any combination of the paging, sync, and traffic channels.
As another example, a typical base station may have 16 W of power that it can potentially use for transmitting all channels on a sector/carrier, and may allocate 15% (2.4 W) of that for the pilot channel, 10% (1.6 W) for the paging channel, and 5% (0.8 W) for the sync channel. When that base station is not serving any mobile stations on active traffic channels on the sector/carrier, i.e. when the sector/carrier is “unloaded,” Ec/Ior would be approximately −3 dB, calculated as 10*log((2.4 W)/(4.8 W)), which, then, would be about as high as Ec/Ior gets. Thus, for reference, anything close to −3 dB may be considered relatively high for Ec/Ior.
And when that same base station is at or near capacity (“fully loaded”), the 15% of its potential sector/carrier power that it is allocating for the pilot channel would shrink from being half of its power output on the sector/carrier (in the unloaded scenario) to, not surprisingly, being about 15% of its power output. This would yield an Ec/Ior of approximately −8 dB, calculated as 10*log((2.4 W)/(16 W)), which, then would be about as low as Ec/Ior gets. Thus, for reference, anything close to −8 dB may be considered relatively low for Ec/Ior. In fact, a typical base station may stop accepting new mobile stations on a sector/carrier once Ec/Ior degrades to about −8 dB. Thus, Ec/Ior can impact sector/carrier capacity as well.
When Ec/Ior is relatively high, this could mean a number of things. For example, there could be only a few mobile stations on the sector/carrier, which would lead to a higher ratio of pilot-channel power allocation to total power allocation (with relatively few traffic channels to which to allocate power). Instead or in addition, it could mean that RF conditions are favorable, such that no (or relatively few) mobile stations are experiencing a poor FFER. In that situation, there would be no (or relatively few) mobile stations inducing the base station to increase power on the traffic channels. This would tend to keep the value of Ec/Ior relatively high. And other possibilities exist as well.
When Ec/Ior is relatively low, this also could mean a number of things. For example, there could be a relatively high number of mobile stations on the sector/carrier, and thus a high number of active traffic channels contributing to a high value of Ior, and thus a low value of Ec/Ior. Instead or in addition, it could mean that RF conditions are poor (e.g., due to terrain, weather, interference, etc.); in that case, mobile stations would likely experience poor FFER, and induce the base station to increase power on the traffic channels, which would contribute to a higher Ior and thus a lower Ec/Ior. And other possibilities exist as well.